Large composite structures incorporating a resin distribution network

ABSTRACT

Large composite structures are produced using a vacuum assisted resin transfer molding process incorporating a resin distribution network. The structure includes cores each having a main feeder groove or channel therein. A resin distribution network is provided adjacent the core surface in fluid communication with the feeder groove. In a first embodiment, the resin distribution network comprises a plurality of microgrooves formed in each core surface. In a second embodiment, the resin distribution network comprises a separate distribution medium surrounding each core. Each core and associated resin distribution network is covered with a fiber material. The dry lay-up is placed against a mold and encapsulated in a vacuum bag. Uncured resin is fed under vacuum directly into the main feeder groove in each core via a fitting through the bag. The resin flows from the main feeder groove through the resin distribution network and outwardly into the fiber material and is allowed to cure. The resin distribution network forms an integral part of the finished structure. The fiber material with cured resin between adjacent cores may comprise various structural members.

This application is a division of application Ser. No. 08/475,849, filedJun. 7, 1995 pending.

FIELD OF THE INVENTION

This invention relates to the production of fiber reinforced resincomposite structures, and in particular to processes for vacuum assistedresin transfer molding of large composite structures.

BACKGROUND OF THE INVENTION

Vacuum assisted resin transfer molding (VA-RTM) has been used to producea number of large, fiber reinforced composite structures such as boathulls which incorporate materials such as foam and balsa cores. Thecores are covered with a fiber reinforced resin. In the VA-RTM process,the reinforcement fiber, such as a fabric or mat, is arranged in asingle sided mold in a dry condition along with the desired corematerials according to the form of the desired finished part. The lay-upis then encapsulated in a vacuum bag and impregnated with resin undervacuum. The resin is allowed to cure.

Various methods have been utilized to introduce and enhance thedistribution of resin through the reinforcement fiber. These methodsinclude the placement of a disposable distribution media over theoutside layer of fabric and the incorporation of holes and/or slotspenetrating through the core to allow resin to flow from the outer tothe inner layer of reinforcement fiber. See, for example, U.S. Pat. Nos.5,316,462 and 4,560,523. A supply groove in a foam core has also beenused in a closed mold resin injection process to facilitate resin flow.See, for example, U.S. Pat. No. 5,096,651.

SUMMARY OF THE INVENTION

The present invention relates to a method for distributing resin duringthe manufacture of large composite structures using a vacuum assistedresin transfer molding (VA-RTM) process and the composite structureproduced by this method. The composite structure is formed from internalcores surrounded by fiber reinforced resin. In one embodiment of theinvention, resin is supplied directly into a network of main feedergrooves which are interconnected to a series of smaller microgroovesformed in the surface of the internal cores. From the feeder grooves andmicrogrooves, the resin flows outwardly from the core to penetrate thereinforcement fiber. In a second embodiment of the invention, a separatedistribution medium is interposed between the internal core and thefiber reinforcement. The resin is supplied directly to one or more mainfeeder grooves in the core surface and penetrates the reinforcementfiber via the distribution medium. Also, the main feeder grooves canextend around the cores to form supply loops, allowing impregnation oftransverse structural members.

With this method, large composite structures which require multiplecores can be formed quickly prior to the gel time of typical vinyl esteror polyester resins, and the amount of resin used can be minimized. Bysupplying the resin directly through the vacuum bag into the feedergrooves, the supply is not limited to a part edge or inlet in a tool.Adjacent cores can be supplied via a single resin inlet. The resindistribution network remains in the finished part, eliminating disposalof distribution materials. The microgrooves are filled with resin aftercuring, thereby increasing interlaminar shear strength and delaminationstrength. Structural features such as shear ties, compression webs, orbeams can be incorporated directly into the composite part during themolding process.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a perspective view of a core for a composite structureaccording to a first embodiment of the present invention;

FIG. 2 is a schematic cross-sectional view of a composite structurebeing formed according to the first embodiment of the present invention;

FIG. 3 is a schematic perspective view of a further composite structurebeing formed according to the present invention;

FIG. 4 is a perspective view of a composite structure being formedaccording to the present invention;

FIG. 5 is a perspective view of a further core for a composite structureaccording to the present invention;

FIG. 6 is a perspective view of a core for a composite structureaccording to a second embodiment of the present invention;

FIG. 7 is a schematic cross-sectional view of a composite structurebeing formed according to the second embodiment of the presentinvention;

FIG. 8 is a schematic cross-sectional view of a composite structurebeing formed using an integrated mold and vacuum structure;

FIG. 9 is a schematic cross-sectional view of a rigid mold and flexiblelid for forming a composite structure; and

FIG. 10 is a perspective view of a core for a composite structure havingmultiple main feeder grooves.

DETAILED DESCRIPTION OF THE INVENTION

A large composite part made according to the present invention includesa core 12, shown in FIG. 1. The core is made from a material able tosupport the pressure of a vacuum. Typical materials include foams, suchas a polyurethane of a polyvinyl chloride, or balsa wood. The core canbe solid or hollow, such as a blown polyethylene. Concrete may also beused. The core is shown as a rectangular block, although otherconfigurations are possible, as discussed further below.

One or more main feeder grooves or channels 14 are provided in thesurface 16 of the core. The main feeder groove may circumscribe theentire core to form a loop. A resin distribution network comprisingchannels of a smaller cross-sectional area than the main feeder grooveis provided in contact with the surface of the core for fluidcommunication with the main feeder groove.

In a first embodiment of the present invention, the resin distributionnetwork is provided in the form of a plurality of microgrooves 18machined in the surface 16 of the core 12, as shown in FIG. 1. Themicrogrooves 18 are generally arranged transversely to the main feedergroove 14. Some of the microgrooves may circumscribe the entire core tocreate a resin flow loop beginning and ending at the main feeder groove.The actual relation of the microgrooves to the main feeder groovedepends on the geometry of the core and the optimization of the resinimpregnation, as discussed further below.

The core 14 with the network of grooves is covered with one or morelayers of a fiber material 20, illustrated schematically in FIG. 2. Thefiber material may be a cloth or mat formed from fibers of glass,carbon, or other suitable material. Depending on the structuralrequirements of the desired finished part, the core may be completelysurrounded with fiber material, or one or more surfaces of the core maybe left free of fiber material. The fiber material may be wrapped in asheet around the core, or individual pieces of fiber material may beapplied to the desired core faces. The fiber may also be supplied in atubular form into which the core is inserted.

A plurality of fiber wrapped cores are arranged to form the desiredfinished part. Although two cores are shown in FIG. 2, the actual numberand arrangement of cores is determined by the desired finished part. Oneor more layers of a fiber material can be wrapped around a plurality ofcores to form an outer skin 22, shown schematically in FIG. 2. Theparticular number of layers of fiber material, the type, and thearrangement depend on the desired finished part and can be readilydetermined by those of skill in the art. A bleeder layer is generallyprovided in the form of a tab 23 extending from an outer fiber layer toa vacuum outlet 25. Peel plies, typically required with prior art vacuumprocesses, are generally not needed with the process of the presentinvention.

The fiber material 24 surrounding and between the cores createsstructural members such as shear ties, compression webs, and beams. Forexample, referring to FIG. 4, a plurality of triangular cores 40 areused to form a deck. The fiber material between adjacent triangularcores forms diagonal structural members 41 that support both compressionand shear forces.

During the lay-up, suitable fittings 26, such as plastic or copper tees,are positioned in the main feeder grooves 14 to facilitate thesubsequent insertion of resin supply tubes 28. One or more fittings maybe positioned in each feeder groove, to accommodate the desired resinflow. The lay-up is placed against a mold 29, and a vacuum bag 30 isthen placed over the lay-up, including the plastic fittings, and sealedto the mold in a manner known in the art, as shown schematically in FIG.2. The vacuum bag is then punctured and the supply tubes 28 are insertedthrough the vacuum bag directly into their respective fittings 26. Thesupply tubes are sealed to the bag to retain vacuum integrity. In thismanner, the main feeder grooves are supplied directly with resin bypenetrating the outer vacuum bag with a supply tube that is inserteddirectly into the groove.

Referring to FIG. 8, the vacuum bag and mold may also be integrated intoa single structure 80 which is rigid enough to retain its shape as amold but flexible enough to collapse against the part upon applicationof a vacuum. For example, the integrated structure 80 may comprise athin gauge steel sheet, such as 0.25 inch or thinner. The cores 82 andfiber material 84, 86, as described above, are encapsulated in the steelsheet. Holes are drilled through the sheet to access the fittings. Resinimpregnation occurs as described above. The integrated structure may beformed of other suitable materials, such as rubber or silicone or a thincomposite sheet material such as a plastic laminated metal.

FIG. 9 illustrates a further mold embodiment in which a rigid mold 90 issealed with a flexible lid 92 formed, for example, from a steel orplastic material. A part, comprising the cores and fiber material asdescribed above, is placed in the recess 94 defined by the rigid mold. Avacuum groove 96 in the lid surrounds the part. Holes are providedthrough the lid or mold to access fittings for resin impregnation asdescribed above. During impregnation of the resin under vacuum, the lidflexes at the edge of the vacuum groove, to allow compaction of thepart.

The resin, such as a polyester, vinyl ester, epoxy, phenolic, acrylic,or bismaleimide, travels relatively quickly through the main feedergrooves 14 and into the microgrooves 18. From the microgrooves, theresin penetrates the fiber material 20, 22. Impregnation results fromresin infusion originating at the core surface 16 and migratingoutwardly to the exterior of the part. The fiber material on adjacentcore surfaces may be impregnated via a main feeder groove in one of theadjacent cores, as indicated in FIGS. 3 and 4.

The cross-sectional area of the main feeder groove and thecross-sectional area and spacing of the microgrooves are optimized toprovide a suitable time to allow the resin to impregnate all of thefiber material before curing without leaving unimpregnated areas. Atypical main feeder groove may have a depth of 0.5 inch and a width of0.5 inch for a cross-sectional area of 0.25 square inches. Typicalmicrogrooves may have a depth of 0.125 inch and a width of 0.125 inchfor a cross-sectional area of approximately 0.016 square inches. Themicrogrooves may be spaced 1.0 inch on center. These dimensions may bemodified to accommodate reinforcement fiber materials of different typesand/or thicknesses. Also, the cross-sectional area of the main feedergrooves may be increased if the part is particularly large to morerapidly distribute the resin to all sections of the part. Similarly,multiple main feeder grooves 14 may be provided in a core 12, asindicated in FIG. 10.

In addition, the cross-sectional area of the main feeder grooves or themicrogrooves may be reduced to create flow restrictions to increaseresin dwell time at a particular area. Resin dwell time may also beincreased by placing a resin "fuse" in the feeder groove whichtemporarily blocks the resin flow. The fuse dissolves after contact withthe resin after a known period of time, which may be set by the lengthof the fuse. For example, with a vinyl ester resin, a styrofoam fuse hasbeen used successfully. The feeder grooves may also terminate toredirect resin flow.

The main feeder grooves 14 allow passage of resin from one core to anadjacent core. Holes may be provided through the cores to connect mainfeeder grooves. Each main feeder groove may be supplied with resinsimultaneously, creating parallel circuits, or in a prescribed sequence,creating series circuits, depending on the geometry and size of the partto be impregnated. Additionally, the main feeder grooves may beindependent of each other, creating separate circuits.

After impregnation, the resin is allowed sufficient time to cure. Oncecured, the microgrooves 18 are filled with solid resin. This resinprovides a lateral locking mechanism which improves the interlaminarshear strength of the bond between the fiber reinforced composite andthe core. The resin remaining in the groove network also increases theforces necessary to delaminate the fiber reinforced face skins from thecore.

The actual arrangement and shape and number of cores depends on thedesired finished part. For example, triangular cores 40 are shown inFIG. 3. The triangular cores may have main feeder grooves 42 provided inat least two surfaces. A central triangular core 44 may have main feedergrooves in three surfaces. Microgrooves are provided in the surfaces asdescribed above. A plurality of triangular cores may be arranged in, forexample, a row to form a deck. In this example, resin, supplied throughtubes 46, is impregnated sequentially beginning at the central core andprogressing toward the edges, as shown by the shaded region 48 in FIG.4.

An arcuate core 50 is shown in FIG. 5. The arcuate core 50 may have amain feeder groove 52 in one surface and a network of microgrooves 54radiating from the feeder groove to circumscribe the core. The arcuatecores may be used to form curved structures such as boat hulls orarches.

In another embodiment of the present invention, illustrated in FIGS. 6and 7, a core 60 is provided with a main feeder groove 62 as describedabove. A distribution medium 64 is then provided adjacent the corefaces. The medium comprises a network of open passageways formed by astructure capable of maintaining the passageways in an open conditionduring application of the vacuum. For example, the medium may compriseintersecting filaments held in spaced relation from the core surface bypost-like members located at each filament intersection, a grid-likestructure of aligned strips, or an open weave fabric. Suitabledistribution media are known for example, from U.S. Pat. Nos. 4,902,215and 5,052,906, incorporated herein by reference. A fiber material 66 isthen wrapped over the distribution media, as described above. Aplurality of cores are arranged to form the desired finished part, and avacuum bag 68 is placed over the cores and fiber material, as describedabove. Resin supply tubes 70 leading from a resin source are insertedthrough the bag 68 and fiber material 66 to fittings 72 in the mainfeeder grooves 62. The supply tubes 70 are sealed to the vacuum bag in amanner known in the art. Resin is fed through the supply tubes to themain feeder grooves. The resin travels relatively quickly through themain feeder grooves and into the distribution media. From thedistribution media, the resin penetrates the fiber material. A suitabletime interval is provided to allow the resin to cure.

Resin distribution media presents a more uniform resin flow front thanthe microgrooves. For this reason, resin distribution media aregenerally preferred for more complicated parts, whereas microgrooves arepreferred to conserve resin, since less resin flows through themicrogrooves.

The invention is not to be limited by what has been particularly shownand described, except as indicated by the appended claims.

We claim:
 1. A unitary composite structure comprising:a core having aperipheral surface and a feeder channel formed to lie across at least aportion of said peripheral surface of said core; a network of groovesformed in said core peripheral surface and extending from said feederchannel in said core peripheral surface, said grooves each having asmaller cross-sectional area than said feeder channel; a fiber materialcovering said core, said feeder channel, and said network of grooves;and a cured resin impregnating said fiber material and substantiallycompletely filling said feeder channel and said network of grooves. 2.The structure of claim 1, wherein said grooves are arranged transverselyto said feeder channel.
 3. The structure of claim 1, wherein at least aportion of said grooves circumscribe said core to form a loop beginningand terminating at said feeder channel.
 4. The structure of claim 1,further comprising a plurality of said cores, each having a peripheralsurface and a feeder channel formed in said surface extending throughouta length of said core, and arranging said cores adjacent to each other.5. The structure of claim 4, wherein said cores are arranged adjacent toeach other with each of said feeder channels generally aligned.
 6. Thestructure of claim 1, wherein said core comprises a foam material. 7.The structure of claim 1, wherein said core comprises balsa wood.
 8. Thestructure of claim 1, wherein said core comprises concrete.
 9. Thestructure of claim 1, wherein said core comprises a block having agenerally rectangular cross-section.
 10. The structure of claim 1,wherein said core comprises a block having a generally triangularcross-section.
 11. The structure of claim 1, wherein said core comprisesa block having an arcuate face.
 12. A unitary composite structurecomprising:a core having a peripheral surface and a feeder channelformed to lie across at least a portion of said peripheral surface ofsaid core; a resin distribution medium laid adjacent said peripheralsurface of said core, said resin distribution medium comprising astructure which defines a plurality of intersecting passages, saidpassages being in fluid communication with said feeder channel in saidperipheral surface of said core; a fiber material covering said core,said feeder channel, and said resin distribution medium; and a curedresin impregnating said fiber material and substantially completelyfilling said feeder channel and said intersecting passages of said resindistribution medium.
 13. The structure of claim 12, wherein said resindistribution medium comprises intersecting filaments held in spacedrelation from the core surface by post-like members located at eachfilament intersection, a grid-like structure of aligned strips, or anopen weave fabric.
 14. The structure of claim 12, further comprising aplurality of said cores, each having a peripheral surface and a feederchannel formed in said surface extending through a length of said core,said cores arranged adjacent to each other.
 15. The structure of claim12, wherein said cores are arranged adjacent to each other with each ofsaid feeder channels generally aligned.
 16. The structure of claim 12,wherein said core comprises a foam material, balsa wood, or concrete.17. The structure of claim 12, wherein said core comprises a blockhaving a generally rectangular cross-section, a generally triangularcross-section, or an arcuate face.
 18. A unitary composite structurecomprising:a core having a peripheral surface and a feeder channelformed to lie across at least a portion of said peripheral surface ofsaid core; a resin distribution network adjacent said peripheral surfaceof said core, said resin distribution network comprising a structuredefining a plurality of intersecting passages, said passages being influid communication with said feeder channel in said peripheral surfaceof said core; a fiber material covering said core, said feeder channel,and said resin distribution medium; and a cured resin impregnating saidfiber material and substantially completely filling said feeder channeland said intersecting passages of said resin distribution medium. 19.The structure of claim 18, wherein said structure defining said passagescomprises a network of grooves formed in said peripheral surface of saidcore extending from said feeder channel, said grooves having a smallercross-sectional area than said feeder channel.
 20. The structure ofclaim 18, wherein said resin distribution structure defining saidpassages comprises a resin distribution medium laid adjacent to saidperipheral surface of said core.
 21. The structure of claim 20, whereinsaid resin distribution medium comprises intersecting filaments held inspaced relation from the core surface by post-like members located ateach filament intersection, a grid-like structure of aligned strips, oran open weave fabric.
 22. The structure of claim 18, further comprisinga plurality of said cores, each having a peripheral surface and a feederchannel formed in said surface extending throughout a length of saidcore, said cores arranged adjacent to each other.
 23. The structure ofclaim 18, wherein said core comprises a foam material, balsa wood, orconcrete.
 24. The structure of claim 18, wherein said core comprises ablock having a generally rectangular cross-section, a generallytriangular cross-section, or an arcuate face.